Various types of optical smoke detectors have been known for some time. The obscuration type detector includes an emitter or lamp for producing light within a chamber, and a sensor (such as a photocell or photodiode) also positioned within the chamber to provide an electrical output signal as a function of the radiation incident on the sensor. As particles of combustion enter the chamber and reduce the amount of radiation impinging on the sensor, the electrical output signal from the sensor is reduced to indicate the presence of smoke in the chamber. Another type of optical detector is the Tyndall effect detector. The Tyndall phenomenon refers to the visible scattering or diffusion of radiation along the beam when particles are struck by the beam. The diffusion or scattering of light produced when particles of combustion enter the beam actually increases the level of radiation impinging on the sensor, thus producing an increase in the information signal utilized to determine the presence of particles of combustion.
Different types of optical detectors have been utilized in recent years. One very effective type of detector utilizing the Tyndall effect is described and claimed in the abandoned application of Richard Roy Saltzgaber entitled "Smoke Detector Chamber and System", filed Apr. 19, 1978, Ser. No. 897,970, and assigned to the assignee of this invention. When utilizing such an arrangement, it is desirable to know that the system is in fact functioning well, including the operability of the emitter and sensor used in connection with the smoke chamber itself.
It is therefore a primary object of the present invention to provide a supervisory or monitoring system for an optical detector, effective to check the operability of the components directly associated with its smoke chamber.
It is also desirable to minimize the spurious triggering of the system by stray signals and likewise to enhance the amplitude of the signal received from the sensor. To this end the smoke detector systems frequently use bandpass filters and amplifiers, or related signal processing arrangements, between the output of the sensor and the point at which the signal is evaluated to indicate the presence of smoke in the chamber. Of course such additional signal processing circuits are also susceptible to aging or other breakdowns.
It is therefore another important object of the present invention to provide a supervisory examination arrangement which, in addition to monitoring the chamber emitter and sensor, similarly checks the effectiveness of the signal processing circuits.
Many of the smoke detector units sold today are powered by batteries. Because the system circuitry is generally comprised of semiconductors there is little drain on the battery from the normal system components. However, if the addition of supervisory circuitry is not carefully done, then the advantages to be obtained by monitoring the system might be offset by adding an undue drain on the battery or other source of energizing potential difference.
It is thus another important object of the present invention to provide a supervisory examining system for a smoke detector which is activated only periodically, to minimize the drain on the battery, but with sufficient frequency to ensure prompt discovery of a failure in the system component.
Yet another important object of the invention is the provision of a supervisory examination circuit which also checks the level of the voltage supplied to the system after the battery voltage drop is considered.